The study seeks to determine the comparability of azithromycin plus atovaquone to TMP/SMX in prevention of serious bacterial infections. It will also determine and compare the long-term safety and tolerance (as defined by the rate of treatment-limiting adverse events) of micronized atovaquone plus azithromycin versus TMP/SMX. For the first 30 patients, the study will examine if there is a likelihood of pharmacokinetic interaction between atovaquone and azithromycin.